Collaborative Research: A global examination of the subduction zone flow field from seismic anisotropy

当构造板块冲回地球时会发生什么？S在俯冲带内部？令人惊讶的是，这个简单的问题仍然是地球科学中尚未解决的基本问题之一。通常使用地震各向异性来研究下行板块(通常称为板块)周围的流动模式，地震各向异性是地幔的一种属性，可以通过记录地震波来检测到。特别是，横波分裂的测量可以用来表征俯冲带的地震各向异性。从这样的测量中越来越清楚地看到，平板上方的二维角流动和平板下方的夹带流动的最简单模型可能是不正确的。然而，各方尚未就替代模式达成共识。在这个项目中，我们将对剪切波分裂观测进行全球调查，目的是了解是什么控制了俯冲带地区的地幔流场。为此，我们将确定描述俯冲带动力学的参数，这些参数似乎对剪切波分裂施加一阶控制。初步工作已经确定了与沟槽迁移速度的大小有关的板面上方和下方的各向异性的系统变化。这导致了一种假设，即三维流动在板块下方占主导地位，并与地幔楔体中的二维角流相互作用。除了系统地评估俯冲带的地震各向异性外，我们还将建立板块上方和下方的地幔流动的实验室和数值模型，以确定横波分裂测量中的流场诊断特征，并探索我们的模型对地幔动力学的意义。该项目是一项跨学科的努力，目的是通过观测地震各向异性和地球动力学模型来理解和描述伴随俯冲而来的地幔流场的特征。随着剪切波分裂作为绘制地幔流动的一种工具的日益普及，现在可以从俯冲带获得大量的数据。因此，现在进行一次分裂观测的全球调查是及时的，目的是了解哪些俯冲参数(如会聚速度、海沟迁移和曲率、下行板块的年龄和扩张历史、板块倾角和形态、地震活动、弧长、覆盖板块厚度和应力以及火山生产)似乎控制着俯冲带流场。根据初步调查，我们假设在板块下方以三维流动为主，该流场与地幔楔形体内的二维角流相互作用。我们将通过实验室和数值模拟研究来补充我们的主要观测地震学目标。这项正向模拟工作将用于验证我们工作模型的预测，制定替代假设，确定对流场的任何二阶影响，并探索我们的工作模型对更大尺度地幔动力学的影响。来自全球许多地区的各向异性约束条件的可获得性，以及地震学观测、实验室和数值模拟的结合表明，下行板块与周围地幔之间相互作用的根本问题的解决是指日可待的。

What happens when a tectonic plate plunges back into the Earth?s interior at a subduction zone? Surprisingly, this simple question remains one of the fundamental unsolved problems in earth science. The flow pattern around the downgoing plate (often called the slab) is typically studied using seismic anisotropy, a property of the mantle that can be detected by recording seismic waves. In particular, measurements of shear wave splitting can be used to characterize seismic anisotropy in subduction zones. It is increasingly clear from such measurements that the simplest model of two-dimensional corner flow above the slab and entrained flow beneath the slab is likely incorrect. However, consensus on an alternative model has not been forthcoming. In this project, we will undertake a global survey of shear wave splitting observations with the goal of understanding what controls the mantle flow field in subduction zone regions. To do this, we will identify parameters that describe subduction zone dynamics that appear to exert a first-order control on shear wave splitting. Preliminary work has identified systematic variations in anisotropy both above and below the slab linked with the magnitude of trench migration velocity. This has led to the hypothesis that 3-D flow dominates beneath the slab and interacts with 2-D corner flow in the mantle wedge. In addition to a systematic evaluation of seismic anisotropy in subduction zones, we will construct laboratory and numerical models of mantle flow above and below the slab to identify diagnostic features of the flow field in shear wave splitting measurements and to explore the implications of our model for mantle dynamics. This project constitutes an interdisciplinary effort to understand and characterize the character of the mantle flow field that accompanies subduction using observations of seismic anisotropy and geodynamical modeling. With the increasing popularity of shear wave splitting as a tool for mapping mantle flow, a copious amount of data from subduction zones is now available. It is timely, therefore, to undertake a global survey of splitting observations with the goal of understanding which subduction parameters (such as convergence velocity, trench migration and curvature, age and spreading history of the downgoing plate, slab dip and morphology, seismicity, arc length, overriding plate thickness and stress, and volcanic production) appear to control the subduction zone flow field. From a preliminary survey, we hypothesize that 3-D flow dominates beneath the slab and that this flow field interacts with 2-D corner flow in the mantle wedge. We will complement our primary observational seismology goals with laboratory and numerical modeling studies. This forward modeling work will be used to validate the predictions of our working model, formulate alternative hypotheses, identify any second-order effects on the flow field, and explore the implications of our working model for larger-scale mantle dynamics. The availability of constraints on anisotropy from many regions around the globe and the combination of seismological observations and laboratory and numerical modeling suggest that a solution to the fundamental problem of interaction between downgoing slabs and the surrounding mantle is within reach.